This invention relates to thermosettable resin compositions. In one embodiment, the invention relates to compositions containing a bismaleimide and a defined class of bisbenzocyclobutenes. In a specific embodiment, the invention relates to copolymers having superior toughness prepared by reacting a difunctional bismaleimide with a bisbenzocyclobutene having an aromatic linking group.
Advanced composites are high-performance materials made up of a fiber-reinforced thermoplastic or thermosettable material. Thermosettable materials useful in advanced composites applications must meet a set of demanding property requirements. For example, such materials optimally have good high-temperature properties such as high (above 200.degree. C.) cured glass transition temperature and low (less than 3%) water absorption at elevated temperature. Such materials also exhibit high mechanical strength, as reflected in measurements of Mode I fracture toughness above 2 MPa.multidot.m.sup.178 and good compression after impact. For ease of processing in preparing prepregs for composite parts, the uncured material will ideally have a low (below 120.degree. C.) melting temperature.
Examples of thermosettable materials useful in advanced composites include epoxy resins, bisbenzocyclobutene resins and bismaleimide resins. Epoxy resins have good processing properties, but generally have relatively low glass transition temperatures and unacceptable high-temperature water absorbance, and they are also generally brittle.
Standard homopolymers of bisbenzocyclobutene resins (as described in the examples of U.S. Pat. No. 4,540,763) are brittle unless very high molecular weight resins are used. If very high molecular weight bisbenzocyclobutene resins are used, however, the materials are difficult to process into fiber-reinforced composites because of their high viscosity. Bismaleimide resins have this same disadvantage of brittleness and further tend to have high melting points and must be used with solvents in order to be readily processable. In addition, cured bismaleimide resins tend to have high (in the 5-7% range) water absorption. Copolymers of bisbenzocyclobutenes and bismaleimides as illustrated in the Example of U.S. Pat. No. 4,730,030 have good mechanical strength and heat resistance but, like the cured homopolymers, lack the necessary toughness for high-performance applications
It is thus an object of the invention to provide a low-melting material which cures to a high-Tg, tough resin having low water absorption. In one embodiment, it is an object of the invention to prepare bisbenzocyclobutene/bismaleimide copolymers having Tg's above 200.degree. C. and Mode I fracture toughness greater than 2 MPa.multidot.m.sup.1/2.